Question

A standard penetration test was conducted at a site 12 m below the earth's surface. The observed SPT number was 26. The water table is at the ground surface, the soil investigation shows specific gravity of solids to be 2.65 and void ratio to be 0.60. The final SPT number is ?

• A. 20
• B. 23
• C. 31
• D. 41

Answer 1

The correct option is C 31

${\gamma }_{sat}=\frac{(G+e)}{1+e}{\gamma }_w$

$=\frac{(2.65+0.6)\times 9.81}{1+0.6}$

$=19.92kN/m^3$

Effective overburden pressure at test level

$\stackrel{-}{\sigma }=19.92\times 12-9.81\times 12$

$=121.32kN/m^2$

(a) Overburden correction:

$N_1=N_0\times \frac{350}{(\stackrel{-}{\sigma }+70)},\stackrel{-}{\sigma }\le 280kN/m^2$
where $N_0$ = Observed SPT value

$N_1$ = Corrected N-value of overburden
$\stackrel{-}{\sigma }$ = Effective stress at level of test
$kN/m^2$

$N_1=26\times \frac{350}{(121.32+70)}=47.56$

(b). Dilatancy correction:

It is applied to the already corrected N-values for overburden pressure. Dilatancy correction is required only if $[N_1>15]$ in saturated fine sand and silt (i.e., when water table is above test level).

Corrected N - value after dilatancy correction

$N_2=15+\frac{1}{2}(N_1-15)$
$=15+\frac{1}{2}(47.56-15)=31.28$